1. Field of the Invention
The present invention relates to a sheet conveying apparatus, an image forming apparatus, and an image reading apparatus and, more particularly, to a construction for correcting a skew of a sheet such as recording paper, original document, or the like which is conveyed to one of an image forming unit and an image reading unit.
2. Description of the Related Art
Hitherto, an image forming apparatus or an image reading apparatus such as copying apparatus, printer, or facsimile apparatus has a sheet conveying apparatus for conveying a sheet such as recording paper, or original document (hereinbelow, also referred to as an original) to an image forming unit or an image reading unit. The sheet conveying apparatus has a skew correcting unit for correcting a skew of the sheet in order to correct a posture of the sheet or adjust a position of the sheet until it is conveyed to the image forming unit or the image reading unit.
As a correcting system of such a skew correcting unit, there is a loop registration system using a registration roller pair. According to such a system, for example, in the case of the image forming apparatus, a front edge of the sheet is hit against a nip of the registration roller pair which is in the stop state, a deflection is formed in the sheet, and thereafter, the sheet leading edge is curved along a roller nip by elasticity of the sheet, thereby correcting the skew of the sheet. In the case of the image forming apparatus, after the skew was corrected as mentioned above, the registration roller pair is rotated at predetermined timing, thereby matching position of the sheet leading edge with that of a front edge of an image.
However, in recent years, in the image forming apparatus or the image reading apparatus, digitization of processes has been progressed. In association with such digitization, for example, in the image forming apparatus, an interval between the sheets (distance between a rear edge of the precedent sheet and a front edge of the subsequent sheet: hereinbelow, referred to as a sheet interval) is narrowed, thereby enabling many sheets to be processed for a short period of time. Thus, an image forming speed can be substantially improved without raising a processing speed for forming the image.
For example, in a conventional analog copying apparatus, in the case of continuously copying, since an optical apparatus for exposing the original has to be reciprocated by a distance corresponding to the number of copy sheets, the sheet interval adapted to copy information of the original has inevitably been determined.
However, since the processes for reading the original and forming the image are digitized, after the original was read once, the image information is electrically encoded and stored into a memory. Upon image creation, the image information in the memory is read out and an image corresponding to the image information is formed on a photosensitive member in the image forming unit by a laser beam or an exposing apparatus such as an LED array. Thus, a mechanical motion of the optical apparatus or the like which is repeated a plurality of number of times becomes unnecessary even in the case of copying a plurality of print copies. Therefore, a time which is required for the sheet registration mentioned above becomes one of large factors upon determination of the sheet interval.
However, since the foregoing loop registration system has the construction in which the deflection is formed in the sheet and the skew of the sheet is corrected by the elasticity of the sheet, in other words, the sheet is temporarily stopped and the skew of the sheet is corrected, a time which is required for the registration becomes long.
Therefore, as a correcting system proposed in order to shorten the time which is required for the registration, there is a system for correcting the skew while conveying the sheet. As such a correcting system, there is an active registration system using: two sensors arranged on a sheet conveying path on a coaxial line which perpendicularly crosses a sheet conveying direction; and a pair of skew correcting rollers which are independently driven. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. H08-108955.
According to such a system, first, an inclination of a front edge of the sheet is detected based on timing when the sheet leading edge traverses the two sensors. After that, by controlling a sheet conveying speed of each of the skew correcting rollers based on the detection of the inclination of the sheet leading edge, the skew of the sheet is corrected.
FIG. 11 illustrates a construction of a sheet conveying apparatus for correcting the skew by such an active registration system. The sheet conveying apparatus has: a registration unit 7 for correcting the skew of the sheet; and a sheet conveying unit 8 which is arranged on the upstream in the sheet conveying direction of the registration unit 7 and has a plurality of rollers 5a and 6a each having a semicircular cross section.
When a sheet edge detecting unit (not shown) detects the sheet leading edge, the semicircular rollers 5a and 6a are rotated a predetermined number of times at the same phase. By this rotation, a sheet S is sent to the registration unit 7. After that, while the sheet is conveyed, the skew of the sheet S is corrected in the registration unit 7.
When the sheet S is sent to the registration unit 7 as mentioned above, the semicircular rollers 5a and 6a are away from rollers 5b and 6b and stopped in the state where the sheet S is not held. By constructing in such a manner that when the sheet is sent to the registration unit 7, the semicircular rollers 5a and 6a do not hold the sheet S as mentioned above, a rear edge side of the sheet is not restricted. Thus, even if the sheet conveying speed is high, the sheet can be continuously conveyed at a minimum interval while maintaining high skew correcting precision.
Another reasonable sheet position matching system for printing at a high speed in which a movement amount of a mass is small has also been proposed. The system executes both of the skew correcting operation of the sheet and the re-positioning operation for matching the side positions of the sheet in the integrated system. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. 2003-054788.
In one of the image forming apparatus and the image reading apparatus, such a skew correction is extremely important to improve one of image forming precision and image reading precision. It is demanded to further improve the skew correcting precision.
Such skew correcting precision largely depends on a “conveying load of the sheet” as a load which the sheet receives from a guide surface of a sheet conveying guide and a “conveying force of the skew correcting roller”. To raise the skew correcting precision, a relation of “conveying load of the sheet<conveying force of the skew correcting roller” has to be always satisfied. If “conveying load of the sheet>conveying force of the skew correcting roller”, the skew cannot be corrected. Even in the case near such a relation, a slip occurs between the sheet and the skew correcting roller and the skew correction is insufficient, thereby deteriorating the correcting precision.
Particularly, in the case of curving a sheet conveying path on the upstream of the skew correcting roller in order to miniaturize the apparatus, a contact pressure of the sheet and the sheet conveying guide is increased by a repulsion of a deflection of the sheet, the conveying load is also increased, and the skew correcting precision deteriorates remarkably.
Further, in association with the spread of the recent color copying apparatuses and the like, what is called thick paper in which a weight per unit area is large is often used. Therefore, a technique for coping with such thick paper is demanded. However, since a rigidity of thick paper is large, the repulsion of the deflection increases and the conveying load increases, thereby further deteriorating the skew correcting precision.
Therefore, there has also been proposed an apparatus in which in order to correct the skew at high precision even in a situation of a large conveying load, a conveying roller pair which is moved in the direction (hereinbelow, referred to as a lateral direction) which perpendicularly crosses the sheet conveying direction is provided, thereby assisting the motion of the skew correction of the sheet. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. H10-175752.
However, in such conventional sheet conveying apparatus and image forming apparatus, there is such a tendency that even when the conveying roller pair which is moved in the lateral direction is provided, in recent years, if a variety of many types of sheets are used, the conveying load cannot be sufficiently reduced and the skew correcting precision deteriorates.
If it is intended to increase the conveying force of the skew correcting roller pair in order to improve the skew correcting precision, the contact pressure of the skew correcting roller pair increases. If the contact pressure of the skew correcting roller pair increases as mentioned above, not only the durability deteriorates but also a large torque is necessary, causing an increase in motor size. Thus, the costs increase and the apparatus size increases.